This invention generally relates to spread spectrum radiocommunication systems and, more particularly, to techniques for efficiently allocating spreading codes used to spread information to be transmitted in such systems.
Cellular radio communication systems have recently been developed that use spread spectrum modulation and code division multiple access (CDMA) techniques. In typical direct sequence (DS) CDMA system, an information data stream to be transmitted is superimposed on a much-higher-symbol-rate data stream sometimes known as a spreading sequence. Each symbol of the spreading sequence is commonly referred to as a chip. Each information signal is allocated a unique spreading code that is used to generate the spreading sequence typically by periodic repetition. The information signal and the spreading sequence are typically combined by multiplication in a process sometimes called coding or spreading the information signal. A plurality of spread information signals are transmitted as modulations of radio frequency carrier waves and are jointly received as a composite signal at a receiver. Each of the spread signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. By correlating the composite signal with one of the unique spreading sequences, the corresponding information signal can be isolated and decoded.
As radiocommunication becomes more widely accepted, it will be desirable to provide various types of radiocommunication services to meet consumer demand. For example, support for facsimile, e-mail, video, internet access, etc. via radiocommunication systems is envisioned. Moreover, it is expected that users may wish to access different types of services at the same time. For example, a video conference between two users would involve both speech and video support. Some of these different services will require relatively high data rates compared with speech service that has been conventionally supplied by radio communication systems, while other services will require variable data rate service. Thus, it is anticipated that future radio communication systems will need to be able to support high data rate communications as well as variable data rate communications.
Wideband DS-CDMA is regarded as one candidate for next generation radiocommunication systems that will supply such high data rate communication services. Despite the relatively high spectral efficiency inherent in wideband DS-CDMA techniques, there may be a need to improve the performance of these techniques in order to provide the required high data rates and quality. One proposed technique for augmenting wideband DS-CDMA systems is to use adaptive antennas to transmit signals in wideband DS-CDMA systems. Adaptive antennas direct signal energy associated with specific mobile units into specific geographical areas so that mobile units outside of those areas are not interfered by that signal energy. The use of adaptive antennas in wideband DS-CDMA systems mitigates the self-interference limitation inherent to CDMA systems to an extent that such systems may instead be limited by the number of available spreading codes.
To Applicants"" knowledge, this problem of code limitation is not widely recognized since CDMA systems have traditionally been unable to tolerate the interference associated with transmitting signals using all of the available codes in a set. To better understand this new problem, it is helpful to understand how code sets are used in conventional CDMA systems.
In traditional DS-CDMA systems, different cells use different sets of codes for communication. By having reasonably low cross-correlation between the codes in one set and the codes in other sets, the available frequency bandwidth can be completely reused in each cell. Each code set typically contains a plurality of orthogonal spreading codes that are used to separate different physical channels within the cell. This helps to reduce the downlink intra-cell interference, especially in propagation environments with few multipath components, but also in other, more time dispersive, environments. The number of available orthogonal codes of a certain length is equal to the length of the code, i.e., for a 64 bit code, there are 64 orthogonal codes in each set.
As mentioned earlier, it has conventionally not been possible to simultaneously use all of the codes in a set to transmit information on the downlink due to the self-interference between the transmissions. However, with the introduction of adaptive antennas the interference in the downlink can be reduced to an extent that it will be possible to significantly increase system capacity. For example, shifting from a conventional one antenna implementation to N antennas can provide a capacity increase on the order of N. Under these circumstances, the number of codes available may be the limiting capacity factor rather than downlink interference.
Accordingly, it would be desirable to create new techniques and systems for allocating codes in a flexible manner that permits code reuse sufficient to exploit the full capacity increase potential available by using adaptive antennas in wideband DS-CMDA systems.
These and other problems associated with previous communication systems are solved by Applicants"" invention, wherein multiple downlink spreading code sets are managed in such a way as to minimize interference between users in radio-communication systems using adaptive antennas. Interference between users is dependent on the antenna gain used for transmission to the users, the transmission power and the cross-correlation between the users"" codes. Thus, the present invention uses knowledge of these parameters in allocating codes to users in a manner intended to reduce interference.
For example, mobile stations having antenna gains that interfere severely with each other can be allocated codes having the best cross-correlation properties, i.e., codes in the same code set. In systems which use direction-of-arrival (DOA) information to determine how to steer the downlink signals toward a particular mobile station, this same DOA information can be used as an indicator of how severely different mobile stations"" antenna gains will interfere with each other. Thus, exemplary embodiments of the present invention can allocate codes from one set to the right half of a cell and codes from another set to the left half of a cell in order to minimize interference. In this way, the relatively high cross-correlation between intra-cell transmissions using codes in different sets is suppressed by the gain of the antenna gain.